1. Field of the Invention
The present invention relates to an electromagnetic switch of a starter which is turned on by using a magnetic attraction force to start driving an engine.
2. Description of Related Art
An electromagnetic switch of a starter has been used to start driving an engine of a vehicle. When a driver turns on an ignition switch, an electric current passes through an exciting coil of an electromagnetic switch, so that the exciting coil generates a magnetic field. A fixed magnetic core of the switch is magnetized by the magnetic field, and a plunger of the switch is pulled toward the magnetized core along an axial direction of the plunger. In response to the movement of the plunger, a movable contact of the switch is moved along the axial direction and comes in contact with fixed contacts. That is, the switch is turned on. Then, electric power is supplied to a motor through the fixed and movable contacts to drive the motor, and a rotary shaft is rotated on its axis by the motor. Further, in response to the movement of the plunger, the rotary shaft is mechanically connected with an engine, and a rotational force of the rotary shaft is transmitted to the engine. Therefore, an operation of the engine is started.
In a prior art, the magnetic core of the electromagnetic switch is integrally formed by performing extrusion for a metallic body. However, because the shape of the core is complicated, a manufacturing cost is undesirably increased. To reduce this cost, a technique has been proposed that a base member and a disc member are separately produced and are assembled into the core. In this technique, the base member having a simple structure is produced by performing extrusion, lathe turning or centering for a metallic body, and the disc member is produced by stamping a thin metallic plate in a pressing process. To preferably lead a movable contact moved by the magnetic core into a center hole of the disc member, it is required to thicken the disc member.
However, because the disc member is produced by performing a drawing press for a thin metallic plate, it is difficult to efficiently produce the disc member, and a powerful pressing machine is required to produce the disc member. To solve this problem, Japanese Translation of PCT No. 2002-524826 discloses a technique that a disc member is made by laminating a plurality of thin metallic plates. In this technique, a base member is forcibly inserted into a center opening of each metallic plate so as to assemble the disc member and the base member held each other into a magnetic core. Therefore, a magnetic core can be obtained at low cost.
However, in this magnetic core, the center openings of the thin metallic plates have easily various sizes. Further, because the opening of each plate is formed by shearing the plate in a pressing process, sheared surfaces of the plates easily form a non-flat surface surrounding a center hole of the disc member. In this case, a holding load added to each plate differs from those of the other plates. Moreover, because a plunger collides with the base member each time the electromagnetic switch is turned on, distortion is frequently generated in the disc member repeatedly receiving a pushing force from the plunger through the base member. Therefore, there is a high probability that the base member is detached from the disc member.
In addition, rigidity of a disc member is generally changed in proportional to both a third power of a thickness of each laminated thin plate and the number of plates forming the disc member. Therefore, rigidity of the disc member formed of a lamination body of thin plates becomes lower than that formed of a single plate which has the same thickness as that of the lamination body. Therefore, because the plunger collides with the base member each time a movable contact of the electromagnetic switch comes in contact with fixed contacts of the electromagnetic switch in response to the turn-on of the electromagnetic switch, there is high probability that the whole disc member having low rigidity is bent or warped due to the repeated collision of the plunger with the base member. In this case, contact of the movable contact with the fixed contacts becomes undesirably unstable.